Starch, a kind of polysaccharide-based natural polymer, owns many advantages such as renewability, biodegradability, high stability, bio-safety and bio-compatibility. Thus, it has been widely applied in many technical fields such as healthcare technology. Starch granules get sticky after gelatinization in water and can dehydrate easily in the air. Most of starch-based adhesives rely on this property, such as what supplied in Chinese patents CN101054501A, CN102493274A, CN102757744A and CN 103897629 A.
Previous reports showed that stable starch elastomers that have excellent swelling property and water-retention capacity could be obtained by adjusting the molecular structure of starch or crosslinking with other macromolecules and polymers. For example, Chinese patent CN1480224A disclosed a hydrogel prepared with starch, polyvinylpyrrolidone, polyving akohol, water-soluble fibrin and water. The optimal weight percent of starch is within 5˜15%. The mixture reveals a quasi-gel state initially, and finish the gelation by the exposure under electron beam or y ray with the absorbed dose at 5˜50 kGy. Lamellar like gel adhesion could be induced when the absorbed dose is higher than 20 kGy, while cream like or semi liquid gel could be formed when the absorbed dose is low. The hydrogel has excellent swelling property and stickiness, with mechanical strength on the order of 10−2 MPa, the stretch ratio at fracture approximately 300%, and rate of water absorption between 1000%˜8000%, which could be used as medical dressing and materials for delayed drug release. Chinese patent CN101982202A disclosed a kind of starch based hydrogel, which contains 10˜30 wt. % starch and 2˜15 wt. % water-soluble polymers. In addition, Salt condensation polymer and cross-linker were added during preparation with deionized water as the solvent. The hydrogel has high swelling degree, with the balanced swelling rate as approximately 300%. In addition, the hydrogel could keep wet, own high transparency, and proper mechanical strength. The Chinese patent CN 103833916 disclosed a method for preparing a starch acrylic acid based composite medical hydrogel. The hydrogel is prepared with gelatinized starch, 15˜20% acrylic acid which is neutralized with alkaline, 12˜16% N, N′-methylene bis acrylic amide, and 20˜40% distilled water. The obtained hydrogel has high color stability, good water retention capability and bactericidal ability, and could be used efficiently for more than 720 h, which is suitable as a composite dressing. The above patents all prepared starch based hydrogel. However, all of them added polymers other than starch during preparation, and used cross-linker. The procedures are complicated and have great influence on the environment. The obtained hydrogels own limited ductility and almost have no conductive capability, which limit their applications in some fields.
The present conductive elastic materials are mainly electronic conductive which are prepared by mixing conductive particles or fibers. The Chinese patent CN 101918495 disclosed an electronic conductive elastomer, which is prepared by dispersing carbon nanotubes (CNTs) in the thermoplastic elastomer. The diameter of CNTs is 30˜300 nm, and the aspect ratio is 10˜100. The thermoplastic elastomer belongs to polyester system. CNTs were added with weight ratio of 0.01˜10 to 100 elastomer. The bulk resistivity of the material is below 1011Ω·cm. The Chinese patent CN 102702662 A disclosed a kind of thermal and electrical conductive elastic material and the preparation method. The material is prepared by adding metal fibers, copper nanoparticles and expanded graphite into the styrene elastomer. And the mineral oil, polyolefin, and antioxidant are added as extra additives. The prepared materials are heat conductive, electronic conductive (volume electronic resistance is approximately 103Ω·cm, soft with low hardness.
In addition to adding conductive elementary substances, the metallic salts could also be added to prepare polymeric electrolytes, which are ionic elastomers, through the complexation of alkali metal ions with the polyether block. The Chinese patent CN 103131165 A disclosed a conductive PA12 elastomer and the preparation method. The elastomer was prepared by adding alkali metallic salts into the PA12 elastomer, with addition materials including antioxidant, heat stabilizer, and conductive agent. The cations in the alkali metallic salts could be Li+, Na+ and K+, and the anions could be Cl−, Br−, I−, and ClO4−. This elastomer own advantages including stable and high conductivity (surface resistance of 102˜103Ω·cm), low cost, good heat resistance. However, this conductive elastomer use thermal plastic elastomer as the substrate, which has limited stretchability, limited its applications.